1. Field of the invention
The present invention relates to ion transport membranes and float glass furnaces.
2. Related art
Nowadays, the glass industry can be divided into four product segments: Flat glass (windows, automobile windshields, and picture glass); Container glass (bottles, jars, and packaging); Glass fiber (insulation/fiberglass, textile fibers for material reinforcement, and optical fibers); and Specialty glasses (pressed/blown glass such as table and ovenware, flat panel display glass, light bulbs, television tubes, and scientific and medical applications.
Glass manufacture, regardless of the final product, requires four major processing steps: batch preparation, melting and refining, forming, and post forming. Batch preparation is the step where the raw materials for glass are blended to achieve the desired final glass product. While the main components in glass are high-quality sand (silica), limestone, and soda ash, there are many other components that can be added. The details of the batch material are well known in the art and need not be discussed here. Once the mixed batch is charged to a melting furnace, melting of the batch may be accomplished in many different types and sizes of furnaces, depending upon the quantity and type of glass to be produced. The melting step is complete once the glass is free of any crystalline materials. Refining (also referred to as fining) is the combined physical and chemical process occurring in the melting chamber during which the batch and molten glass are freed of bubbles, homogenized, and heat conditioned. After refining, the molten glass is sent to forming operations. Forming is the step in which the final product begins to take shape, and may involve casting, blow forming, sheet forming, fiberization, or other processes. Forming processes vary widely, depending on the type of glass being manufactured. Some products require post-reforming procedures, which vary widely depending upon the products. The post-reforming procedures may include processes that alter the properties of the glass, such as annealing, tempering, laminating and coating.
To make glass, one necessary condition is to supply high-temperature energy to the process. This can be done by flames generated through burners installed above the glass bath or electrically by using electrodes submerged in the glass bath. Flames of course result from the combustion of a fuel (such as natural gas, oil, or coal) and an oxidant (such as air, oxygen-enriched air, or high purity oxygen). Most glass furnaces use air as an oxidant. In some conditions, high purity oxygen is preferred despite the extra cost due to the oxygen price. Reducing pollutants emissions (such as NOx, Sox) or greenhouse gases (CO2), fuel consumption, and capital investment are typical advantages associated with the use of high purity oxygen.
In the industrial gas industry, large amounts of oxygen are typically supplied by one of our methods: bulk liquid tanks which are filled regularly by bulk liquid trucks, vacuum swing adsorption (VSA) which provides low purity oxygen at low pressure, an oxygen pipeline, or a dedicated air separation unit. Supply by bulk liquid tanks is not practice for furnace powers higher than 2 MW (≈7 MMBtu/hr) due to the sheer number of truck deliveries needed. It should be noted that flat glass furnaces are operated usually at 35 MW. VSAs are limited in capacity (10 MW is maximum allowable level). While oxygen pipelines are ordinarily considered the most appropriate, their application to glass furnaces is limited to where they are located. Additionally, the risk of pipeline failure is always taken into account. Finally, a dedicated ASU is ordinarily considered an oversized solution for 35 MW glass furnaces.
Some have proposed to reduce the fuel and/or oxygen requirements of glass furnaces by preheating air or oxygen. In air-fired furnaces, flue gases are used to preheat air to 600° C. (1100° F.) or up to 1250° C. (2300° F.). In oxy-fired furnaces, such a technique is difficult to implement because pure oxygen is a very hazardous material and ignition of a mixture with oxygen can jeopardize the furnace. One particular solution from Air Liquide involves the preheating of oxygen and natural gas with hot flue gas via an intermediate heat exchange fluid of air. In this manner, oxygen may be preheated to 550° C. and natural gas to 450° C. Fuel savings of about 10% can be realized with implementation of this technology.
Some glassmakers and engineering companies sells boilers and power station using the fumes energy but the yield is still too low to be really profitable.
Another way of producing oxygen on-site that has not yet been commercially implemented is the use of high temperature ion transport membranes (ITMs). ITMs are particular types of solid electrolytes that are inorganic crystalline materials that, while being impermeable to gases, have the property of conducting oxygen ions (O2−) or protons (H+) through vacancies in its crystalline structure. In order to maintain electric charge neutrality, certain solid electrolyte membranes must include a separate electron-conductive path. Ones that conduct oxygen ions are called oxygen transport membranes while ones that conduct protons are called hydrogen transport membranes. Other solid electrolyte membranes are made of materials that, at elevated temperatures, can simultaneously conduct oxygen ions and electrons or simultaneously conduct protons and electrons. Examples of these oxygen ion conductive materials include certain perovskites such as LaxSr1-xCoO3-y, LaxSr1-xFeO3-y, and LaxSr1-xFeyCo1-yO3-z are examples of mixed conductors. One example of a proton conductive material is a cermet, a composite of metal and sintered ceramic. Other examples of proton conductive materials include the single-phase mixed metal oxide materials of the formula: AB1-xB′xO3-y wherein A is selected from Ca, Sr or Ba ions, B is selected from Ce, Zr, Ti, Tb, Pr, or Th ions, B′ is selected from Yb, In, Ru, Nd, Sc, Y, Eu, Ca, La, Sm, Ho, Tm, Gd, Er, Zr, Gb, Rh,Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Al, Ga, or In ions (or combinations thereof), x is greater than or equal to 0.02 and less than or equal to 0.5, and y is such that the electrical neutrality of the crystal lattice is preserved. These oxygen ion or proton conductive membranes are often called mixed oxide conducting membranes.
Other terms used to describe these membranes include mixed ion and electron(ic) conducting membranes, mixed proton and electron(ic) conducting membranes, ion transport membranes, oxygen transport membranes, hydrogen transport membranes, solid state membranes, mixed conducting metallic oxide, and mixed conducting multicomponent metallic oxide membranes. Regardless of the name utilized, these materials have the ability to transport oxygen ions (O2−) or protons (H+) through their crystalline structure.
Using oxygen conductive mixed oxide conducting membranes as an example, at elevated temperatures, the mixed oxide conducting material contains mobile oxygen ion vacancies that provide conduction sites for transport of oxygen ions through the material. The membrane is in part driven by a difference in oxygen partial pressure across the membrane. When the surface of the membrane is exposed to the relatively higher O2 partial pressure gaseous atmosphere, the molecular oxygen in the gaseous atmosphere adjacent the surface reacts with electrons and the oxygen vacancies in the crystalline structure of the material to product oxygen ions O2−. The oxygen anions diffuse through the mixed conductor material to the opposite surface of the membrane which is exposed to the relatively lower O2 partial pressure. At the opposite surface, the oxygen anions give up their electrons and form molecular oxygen. The molecular oxygen then diffuses into the gaseous atmosphere adjacent the surface of the membrane exposed to the lower O2 partial pressure gaseous atmosphere. These materials transport oxygen ions selectively, and assuming a defect-free membrane and lack of interconnecting pores, they can act as a membrane with an infinite selectivity for oxygen.
Proton conductive mixed oxide conducting membranes operate in much the same way and are similarly in part driven by a difference in hydrogen partial pressure across the membrane. When the surface of the membrane is exposed to the relatively higher H2 partial pressure gaseous atmosphere, hydrogen molecules disassociate into protons and electrons which migrate through the membrane to the opposite surface where they recombine into hydrogen molecules. The thus-formed hydrogen molecules then diffuse into the gaseous atmosphere adjacent the membrane surface. Similar to oxygen conducting mixed oxide conducting membranes, these proton conducting membranes offer the possibility of infinite selectivity for hydrogen.
In oxygen transport membranes, air is compressed to about 16 bars, heated to 900° C., and fed to the ITM and hot oxygen permeates through the membrane. The permeate pressure must be kept low in order to provide the necessary oxygen partial pressure driving force across the membrane. A recovery of 50% to 80% of the oxygen in the air is considered feasible. ITMs can thus provide oxygen at a temperature of around 900° C. and at a low pressure ranging from 0.5 bara to 2 bara. The product oxygen can be withdrawn at different pressures, such as 0.5 bara, 0.7 bara, 1.1 bara, or 2.2 bara in order to minimize recompression energy. Oxygen produced at high temperature and low pressure does not require further preheating and is suitable for use in the glass furnace. In addition to the product oxygen, a hot non-permeate gas containing nitrogen and non-recovered oxygen is available at 900° C. and 16 bars. The use of the non-permeate gas is a challenge since it may drag the efficiency down if not treated properly.
In flat glass furnaces, melted glass exits the glass bath at around 1100° C. (2220° F.). At one meter wide and ten centimeters thick, the melted glass flows on top of a tin bath. At this temperature, the tin reacts in presence of O2 in the atmosphere to produce SnO vapor. Because of high levels of SnO equilibrium vapor, a significant quantity of SnO can be formed through condensation of the SnO vapor at the cold spots in the roof. This solid condensate falls down on the glass when it grows to a certain size and mechanically damages the glass. Thus a reductive atmosphere is typically used in order to protect the Sn from oxidation. The atmosphere above the tin bath is ordinarily composed of 90% N2 and about 10% H2. About 1000 Nm3/h of N2 and 100 Nm3/h of H2 are needed to fulfill the space above the glass. Such high quantities are usually provided by pipe.
While each of the above technologies is interesting in its own right, glass manufacturers utilizing float glass furnaces still strive to reduce costs. Thus, there is a need to provide a new technology which preserves the above advantages while driving down costs.